Pavement road markers have been used for many years to delineate lanes on highways. Road markers typically have one or two retroreflective surfaces mounted in a plastic shell. The plastic shell is typically potted or filled with a resinous material to give strength and weight to the shell. Retroreflective lenses are molded of an acrylic plastic. The lens may be covered with glass to minimize scratching.
The lenses have cube corners formed on one side. The cube corners are metallized to increase the reflectivity of the lenses. The cube corners are metallized because a highly reflective surface is needed to perform a good specular reflection (like mirrors), in combination with cube corners, retroreflectivity is performed, otherwise having in contact the back of the lens and the filler having not highly reflective nature, specular reflection will be not present. Another way to have reflection is a phenomena known as Total Internal Reflection (TIR) http://en.wikipedia.org/wiki/Total_internal_reflection. TIR doesn't need a highly reflective surface, just need a combination of incidence angle and refractive index (plexiglass about 1.45/air 1.000293). TIR combined with cube corners geometry perform retroreflectivity. However, the plating process is expensive and time consuming.
The tires of vehicles pass over the road markers when the vehicles change lines. Thus, the road marker is subjected to substantial impact from vehicles, particularly heavy vehicles such as trucks. Accordingly, the road marker must be strong enough to withstand the force, and the surface of the lens must be sufficiently tough to resist scratching from stones and grit adhered to the tire tread.